Mishkat al-Masabih #602

Ibn ‘Umar said that when the Muslims came to Medina they gathered and sought to know the time of prayer, but no one summoned them. One day they discussed the matter, and one of them said, “Use something like the bell of the Christians." Another said, “Use a horn like that of the Jews." But when ‘Umar said, “I suggest that you send a man to announce the prayer," God’s Messenger said, “Get up, Bilal, and summon to prayer." (Bukhari and Muslim.)